Various modes of glow plugs used to preheat a diesel engine and for other purposes have heretofore been known in the related art. A glow plug 101, a general view and a longitudinal sectional view of which are shown in FIG. 13 and FIG. 14, respectively, is included as an example of the known glow plugs. This glow plug 101 is provided with a cylindrical sheath tube 103 closed at its front end 103S and opened at its base end 103K, and a cylindrical metal shell 105 covering the sheath tube 103 on the side of the base end 103K. Of these parts, the sheath tube 103 projects, at its portion having a length of about 36 mm from its front end 103S, from the front end of the metal shell 105, and an outer diameter of this portion of the sheath tube is at a uniform value of 5.0 mm.
Inside of the sheath tube 103, a heating coil 107 is arranged along the axis of the sheath tube 103, as shown in the partial enlarged sectional view of FIG. 15. A front end portion 107S of this heating coil 107 is electrically connected with the front end 103S of the sheath tube 103. Into the portion of the sheath tube 103 on the side of the base end 103K, there is inserted a part of a rod-shaped electric terminal shaft 109 along the axis of the sheath tube 103. A front end portion 109S of the electric terminal shaft 109 is electrically connected with a base end portion 107K of the heating coil 107 in the vicinity of the front end of the metal shell 105. Furthermore, the interior of the sheath tube 103 is filled with magnesia powder 111 or insulating powder.
When such glow plug 101 is fixed to a cylinder block of a diesel engine with a voltage being applied to the electric terminal shaft 109 from a vehicle-mounted battery as a power source, an electric current flows from the electric terminal shaft 109 to the cylinder head (or an engine block) through the heating coil 107, sheath tube 103 and metal shell 105. As a result, a high current flows to the heating coil 107 to raise its temperature, so that the sheath tube 103 is heated substantially as a whole at its portion projecting from the metal shell 105.
However, this glow plug 101 of the related art takes time to be heated to a temperature needed for preheating the diesel engine. For example, it takes around 15 seconds to heat up to about 1,000° C. the portion of the sheath tube 103 in the vicinity of the front end 103S.
The present invention has been made in view of such circumstances, and has an object to provide a glow plug, the temperature of which can be speedily raised, a structure for mounting the glow plug, and a suitable process for manufacturing the glow plug.